Ductwork clash detection

Why does ductwork drive so many clashes?

Ductwork is the largest physical service in most ceilings, a 24x12 supply trunk takes more plenum than most pipes and conduits combined. Ductwork also branches frequently, has insulation that increases outside dimensions beyond the duct's nominal size, and serves equipment with tight elevation requirements. Most above-ceiling clashes involve ductwork.

What are the most common ductwork clashes?

The most common ductwork clashes are trunks crossing beams without openings, insulated ducts exceeding the plenum, and ducts occupying the same elevation as sprinkler mains.

• Supply or return ductwork crossing structural beams without coordinated openings
• Insulated duct dimensions exceeding available plenum
• Ductwork crossing fire sprinkler branch lines at the same elevation
• Return air plenum interrupted by hung piping or conduit
• VAV box installed above a ceiling that lacks the required service access
• Diffuser locations conflicting with sprinkler heads or light fixtures
• Duct hangers conflicting with cable tray or other suspended utilities
• Equipment dimensions on shop drawings larger than design drawings

How should you model ductwork for clash detection?

Effective clash detection starts with accurate modeling. Specific items to verify in the model:

• Ducts modeled at outside dimensions including insulation thickness
• Hangers and supports modeled, not just ducts
• VAV boxes, fire/smoke dampers, and other in-line components included
• Service clearance volumes modeled for VAVs and similar equipment
• Diffuser neck height and trim dimensions modeled to ceiling face
• Risers modeled with the actual transitions, not just rectangular boxes
• Equipment listed with actual manufacturer dimensions, not generic library blocks

What is the ductwork clash-detection workflow?

Run ductwork clash detection at design development against structure, again at 60 percent CDs against all MEP, and once more at 95 percent CDs before fabrication.

• Run clash detection at DD with ductwork vs. structure to verify available plenum
• Run at 60% CD with ductwork vs. all MEP and structure
• Run at 95% CD with ductwork vs. everything including ceiling fixtures
• Categorize clashes by severity: hard clash, soft clash (clearance), or acceptable
• Assign each clash to a discipline owner for resolution
• Re-run after resolution to verify the fix didn't create a new clash

What should you field-verify before sheet-metal fabrication?

Even with BIM, field conditions don't always match the model. Verify the critical items on-site before sheet metal is fabricated:

• Existing structure dimensions and elevations
• Ceiling height vs. designed clear height
• Beam web depths and stiffener locations
• Existing piping and conduit not in the model (typical on renovations)
• Equipment locations vs. designed locations

How do you resolve hard ductwork clashes?

Resolve a hard ductwork clash by rerouting the more flexible service, raising a soffit, or dropping a beam, then documenting the agreed path on the coordination set.

• Reroute the smaller / more flexible service (typically conduit before duct)
• Raise the ceiling or modify the soffit in localized areas
• Drop a beam or add a coordinated opening (requires structural review)
• Resize ductwork (slower air velocity may allow smaller duct)
• Relocate equipment to a less congested area

How do you resolve soft ductwork clashes?

Resolve soft ductwork clashes by lowering or rerouting duct, shrinking a run, or moving the conflicting pipe, cable tray, or structure.

• Confirm code or manufacturer minimum clearances
• Verify maintenance access is preserved
• Document any clearance variance approval from the AHJ or designer